The control of the radial clearance between the tips of rotating blades and the surrounding annular shroud in axial flow gas turbine engines is one known technique for proving engine efficiency. By reducing the blade tip to shroud clearance, designers can reduce the quantity of turbine working fluid which bypasses the blades, thereby increasing engine power output for a given fuel or other engine input.
"Active clearance control" refers to those clearance control arrangements wherein a quantity of cooling air is employed by the clearance control system to regulate the temperature of certain engine structures and thereby control the blade tip to shroud clearance as a result of the thermal expansion or contraction of the cooled structure. It is a feature of such active clearance control systems that the cooling air flow may be switched or modulated responsive to various engine, aircraft, or environmental parameters for causing a reduction in blade tip to shroud clearance during those portions of the engine operating power range wherein such clearance control is most advantageous.
A reduction of blade tip to shroud clearance must be achieved judiciously. For example, overcooling the turbine case supporting the annular shroud such that the shroud interferes with the rotating blade tips results in premature wear of the shroud or abrasion and damage to the blade tips. It is therefore important that the reduction in blade tip to shroud clearance achieved by such clearance controls systems must be designed so as to avoid the occurrence of blade tip and shroud interference which may ultimately cause deterioration of overall engine operating efficiency, or worse, damage to the engine internal components.